The present invention relates to a method of manufacturing granular detergents possessing improved fluidity, hygrocaking property and compression caking property wherein a detergent slurry and an aluminosilicate slurry are subjected separately to spray-drying within the same drying space.
As is generally known, not only are phosphate superior as builders for granular detergents, but also they play an important role in maintaining satisfactory powder properties, such as fluidity, hygrocaking property and compression-caking property, of granular detergents.
However, now that a problem has arisen in eutrophication of rivers and lakes caused by phosphates, there is a great demand for granular detergents having a low phosphate content. Accordingly, the powder properties of granular detergents show a tendency to considerable deterioration. This is due to the reason that, even though the deterioration of detergency caused by reducing the amount of a phosphate-type builder mixed therein may be somewhat made up for by the use of an alkali salt of silicic acid, carbonic acid or bicarbonic acid and by increasing the amount of the surface active agent, such alkali salt does not contribute very much to an improvement in the powder properties of granular detergents, as compared with phosphates.
As a means for improving the powder properties of granular detergents, the art of applying a certain coating onto the surface of particles of granular detergents is disclosed in U.S. Pat. Nos. 3,925,226, 3,950,275 and 3,989,635. The art disclosed in U.S. Pat. No. 3,925,226 is a method comprising wetting the surfaces of particles of granular detergents with a lower alcohol or a solution of a perfume, adding a metallic soap powder to the wet particles and coating the surface of the particles with a metallic soap powder by utilizing the adhesive power of the lower alcohol or the solution of perfume. The art disclosed in U.S. Pat. No. 3,950,275 is a method comprising wetting the surfaces of particles of granular detergents with water or an aqueous solution of a specific binder and attaching a specific powdery builder to the wet surface of the particles. The art disclosed in U.S. Pat. No. 3,989,635 is a method comprising spraying two kinds of aqueous solutions containing specific compounds, respectively, onto particles of granular detergents and allowing the compounds contained in these two kinds of aqueous solutions to react with each other thereby forming a water-insoluble film on the surfaces of the particles.
The foregoing methods are all intended to improve the powder properties by initially manufacturing a granular detergent by a spray-drying method and then applying an after-treatment to the granular detergent. Therefore, the finally obtained granular detergent is admittedly provided with satisfactory powder properties, but the granular detergent obtained only spray-drying is not provided with satisfactory powder properties. It is of course desirable that a granular detergent in the form of the final product should be provided with satisfactory powder properties, but it is more desirable that it should already be provided with satisfactory powder properties when it is discharged from the spray-drying tower.
There has recently been advocated the use of an aluminosilicate, which is insoluble in water but is capable of blocking calcium ions, as a builder for granular detergents. West German OLS No. 2,529,685 teaches a method of manufacturing granular detergents containing an aluminosilicate by subjecting a detergent slurry to spray-drying within a zone where particles of the aluminosilicate are dispersed. However, this method has a drawback that inasmuch as a relatively large quantity of aluminosilicate particles is directly introduced into the spray-drying tower, the amount of the aluminosilicate particles entrained in the drying air-stream discharged out of the drying tower is great.
The present inventors have found that fine aluminosilicate particles are very effective for improving the powder properties of granular detergents. They have also found that, when an aqueous slurry of fine aluminosilicate particles is subjected to spray-drying, the resulting dry granules are respectively agglomerates of several aluminosilicate particles, but these agglomerates are easily broken down into the individual fine particles in the original state even by weak shocks.